1. Field of the Invention
This invention relates to pneumatic tires, and more particularly to an improvement in an effective reinforcement of a crown portion in the tire.
2. Related Art Statement
In general, the crown portion of the pneumatic tire has exclusively been reinforced by a belt comprised of at least two belt layers, which can not still emerge from problems such as separation failure and the like.
In this connection, Japanese Patent laid open Nos. 54-126306 and 55-132307 disclose that circumferential tension is born by a laminated assembly of cord layers called a limit block or limit assembly to mitigate the circumferential tension acting to the belt, resulting in the prevention of separation failure. In fact, however, there is a fear of causing separation failure between the limit blocks because it is required to laminate cord layers, whose cords are crossed with each other at a very shallow inclination angle with respect to the equator of the tire in order to obtain a high rigidity along the circumference of the tire.
On the other hand, the separation failure can be avoided by using a cord strip having a cord angle of 0.degree. instead of the limit block because the cut free end of the cord is not formed in case of using the strip. However, when using such a strip, the enlargement of the belt in circumferential direction is suppressed in a vulcanizer for green tire, which causes the serious difficulty in the manufacture of tires. Moreover, the twisting of the cord used as a reinforcing element for the strip having a cord angle of 0.degree. is possible to have a structure that it easily elongates only at the initial curing stage and provides a given rigidity after curing. In such a cord, however, the twisting pitch becomes short, which is disadvantageous in view of the productivity, and the cord strength is lowered due to the twisting, so that it is obliged to increase the end count in the strip considering a fear of so-called cut burst in the tire, resulting in the increase of tire weight. Furthermore, it is impossible to use a monofilament as a cord material.
Japanese Patent laid open No. 57-209403 discloses a technique for reinforcing a tire with a protect layer comprised of wavy cords, wherein a wavy body composed of fibers or metal cords having an extremely low elongation is arranged as a protect layer in addition to a breaker comprised of at least two layers. However, there is no description on the effect of the protect layer as a crown reinforcing layer in this technique.